Solid electrolytic capacitors with a very large active capacitor area, which are therefore of a small design suitable for mobile communications electronics, are predominantly those with a niobium or tantalum pentoxide barrier layer applied to an appropriate conductive carrier, utilizing the stability of said barrier layer (“valve metal”), the comparatively high dielectric constants and the insulating pentoxide layer producible with a very homogeneous layer thickness by means of the electrochemical generation method. The carriers used are metallic or conductive lower oxidic (suboxide) precursors of the corresponding pentoxides. The carrier, which simultaneously constitutes a capacitor electrode (anode), consists of a highly porous, spongelike structure which is produced by sintering ultrafine primary structures or already spongelike secondary structures. The surface of the support structure is oxidized electrolytically (“formed”) to the pentoxide, the thickness of the pentoxide layer being determined by the maximum voltage of the electrolytic oxidation (“forming voltage”). The counterelectrode is obtained by impregnating the spongelike structure with manganese nitrate, which is converted thermally to manganese dioxide, or with a liquid precursor of a polymer electrolyte or of a polymer dispersion of a conductive polymer and polymerizing, for example, PEDT. The electrical contacts to the electrodes are produced on one side by a tantalum or niobium wire incorporated by sintering in the course of generation of the carrier structure and, on the other side, by the metallic capacitor shell insulated from the wire.
The capacitance C of a capacitor is calculated by the following formula:C=(F·ε)/(d·VF)where F denotes the capacitor surface area, ε the dielectric constant, d the thickness of the insulator layer per volt of forming voltage and VF the forming voltage.
The sintering of ultrafine primary and/or secondary structures creates a very large active capacitor surface area, but also forms closed pores whose surface is inactive. The closed pores therefore reduce the volume-based capacitance of the capacitors produced from the powders. In the case of use of secondary structures without closed pores, owing to the higher volume-based capacitance, higher sintering temperatures can be used in the production of the anode bodies without loss of capacitance, which in turn leads to an enhancement of the sinter necks and to better wire connection compared to the use of conventional powders. Better wire attachment and thicker sinter necks results in a more stable anode body and a better leakage current, ESR and surge performance of the capacitor.
It is therefore desirable to minimize the number and the volume of closed pores in the capacitor.
One measure of the open pore level of a capacitor anode and of the secondary structures for use for capacitor production (agglomerate powder) is the skeletal density thereof, which is defined as the ratio of the mass of the sinter body to the sum of volume of the solids content and volume of the closed pores. The skeletal density of the anode structures is measured by means of mercury intrusion porosimetry, also known as mercury porosimetry. The customary sintering processes to obtain capacitor anodes achieve skeletal densities of 80 to 88% of the theoretical solid material density.
Processes for influencing the pore structure of capacitor anodes of niobium or tantalum to obtain broad or bimodal pore size distributions have already become known, in which so-called pore formers are used during the sintering step. EP 1291100 A1, WO 2006/057455 describe pore formers used which are organic substances which decompose or evaporate in the course of heating to the sintering temperature, or metals or metal oxides or metal hydrides removable from the sintered structure by acid leaching after the sintering step. DE 19855998 A1 describes gaseous pore formers, by means of which adhesively bound highly porous agglomerates are obtained, which essentially maintain their porosity in the course of sintering.
In these processes, the pore formers are used at relatively late process stages, in which sintered agglomerates with closed pores are already present, such that there is no effective prevention of the formation of closed pores.
When organic pore formers are used, the contamination of the capacitor anode body with carbon is moreover disadvantageous. When metals or metal compounds are used, in addition to possible contamination, a considerable level of effort is required to remove pore formers from the sintered structures.